Audience check: would people reading this know (or know how to find out easily) which platforms lack it and how to work around that?

This is a bit confusing since calling stack_save() or stack_save_td() would be "creating" a trace per the previous sentence. I'd maybe remove the previous sentence and say something to the effect of, "A struct stack can be defined on the stack and initialized using stack_zero(), or dynamically allocated using stack_create().` etc.

Good point.

I also want to clarify that initialization is not needed before stack_save*(), since there are comments in multiple consumers asking this question.

For platform support, yes, I think so. I had never looked at this implementation before, and I found the answer in about ten seconds. A workaround really depends on the context, so that seems out of scope here.